$DF11/57105/MMC+1: Control Register
+-------+---------------------------------------------------------+
| Bit 7 | Unused |
| Bit 6 | 0 = SPI write trigger mode, 1 = SPI read trigger mode |
| Bit 5 | 0 = allow external ROM when MMC64-BIOS is disabled, |
| | 1 = disable external ROM |
| Bit 4 | 0 = Normal Operation, 1 = Flash Mode (*) |
| Bit 3 | 0 = clock port at $DE00, 1 = clock port at $DF20 |
| | Shadow: 0 = MMC64 active, 1 = MMC64 disabled (**) |
| Bit 2 | 0 = 250KHz transfer, 1 = 8MHz transfer |
| Bit 1 | 0 = Card selected, 1 = Card not selected |
| Bit 0 | 0 = MMC64-BIOS active, 1 = external ROM active |
+-------+---------------------------------------------------------+
Notes: (*) Bit 4 can only be written when flash jumper is set
(**) The shadow of Bit 3 can only be modified after
unlocking at $DF13